PURPOSE: To fill the gap between a frame and an armature core with a magnetic fluid in such a manner as to exclude mechanical restriction of the armature core on thermal expansion to conduct heat exchange through the magnetic fluid.

CONSTITUTION: An armature core 2 is supported expandably and compressively between the stator frames 9 via a plurality of bars 10 both ends thereof being coupled with the stator frames 9 fixed to a frame 3, non-magnetic seals 7 having elasticity are provided at the internal side of the stator frames, a gap between the seals 7 is filled with a magnetic fluid 8, and when a generator operates, the magnetic fluid 8 is stirred with a rotating magnetic field leaked from the armature core 2 to cool the armature core 2 through heat exchange between the frame 3 and armature core 2. Even while the generator stops operation, the magnetic fluid 8 remains within the seals 7.